tests: sort import lines in dumbhttp.py

This series is an attempt to enable import-checker.py for tests/**.py, but
it turned out not easy. Since many tests have been ported to absolute_import
without the coverage, import-checker.py reports a lot of errors right now.

Should we enable import-checker.py without fixing all of them so that we
won't get more errors?

tests/dumbhttp.py

@@ -6,10 +6,10 b' from __future__ import absolute_import'
 Small and dumb HTTP server for use in tests.
 """
 
+import BaseHTTPServer
+import SimpleHTTPServer
 import optparse
-import BaseHTTPServer
 import signal
-import SimpleHTTPServer
 import sys
 
 from mercurial import (